__global

Description

The address space qualifier may be used in variable declarations to specify the region of memory that is used to allocate the object. The C syntax for type qualifiers is extended in OpenCL to include an address space name as a valid type qualifier. If the type of an object is qualified by an address space name, the object is allocated in the specified address name; otherwise, the object is allocated in the generic address space.

The address space names without the __ prefix i.e. global, local, constant and private may be substituted for the corresponding address space names with the __ prefix.

The generic address space name for arguments to a function in a program, or local variables of a function is __private. All function arguments shall be in the __private address space.

__kernel function arguments declared to be a pointer of a type can point to one of the following address spaces only: __global, __local or __constant. A pointer to address space A can only be assigned to a pointer to the same address space A. Casting a pointer to address space A to a pointer to address space B is illegal.

The __kernel function arguments declared to be of type image2d_t or image3d_t
always point to the __global address space.

There is no generic address space name for program scope variables. All program scope variables must be declared in the __constant address space.

The __constant or constant
address space name is used to describe variables allocated in
global memory and which are accessed inside a kernel(s) as read-only variables. These read-
only variables can be accessed by all (global) work-items of the kernel during its execution. Pointers to the __constant address space are allowed arguments to functions (including __kernel functions) and for variables declared inside functions. Variables allocated in the __constant address space can only be defined as program scope variables and are required to be initialized.

Writes to variables declared with the __constant address space qualifier in the OpenCL program source should be a compile-time error.

All string literal storage shall be in the __constant address space.

NOTE: Any argument to a kernel that is declared with the __constant address space qualifier counts as a separate constant argument. The relevant device capabilities CL_DEVICE_MAX_CONSTANT_ARGS and CL_DEVICE_MAX_CONSTANT_BUFFER_SIZE are described in the table for clGetDeviceInfo.

Variables inside a function or in program scope can also be declared with the __constant address qualifier. Implementations are not required to aggregate these declarations into the fewest number of constant arguments. This behavior is implementation defined.

Thus portable code must conservatively assume that each variable declared inside a function or in program scope with the __constant qualifier counts as a separate constant argument.